(c) A manufacturer of precision screws has two main lines, wood screws and metal screws, which it sells for 20 and 25 respectively per box. The material costs for each box are 10 and 8 respectively and overhead costs are 5000 per week. All the screws have to pass through a slotting and threading machine. A box of wood screws requires 3 minutes on the slotting machine and 2 minutes on the threading machine, whereas a box of metal screws requires 2 minutes on the slotting machine and 8 minutes on the threading machine. In a week, each machine is available for 60 hours. The company wishes to maximise its weekly earnings. Find how many boxes of each type of screws should be produced per week when the sales potential for both types exceeds the maximum production rate. (11 Marks]